German Patent 199 03 439 discloses a method and a device for controlling an exhaust-gas aftertreatment system, whereby the exhaust-gas aftertreatment system includes at least one catalytic converter, in which nitrogen oxides are reduced by a reducing agent. To do so, a reducing agent is supplied to the exhaust-gas aftertreatment system and/or to exhaust gases upstream from the exhaust-gas aftertreatment system. The quantity of reducing agent is determined on the basis of the mass flow of the nitrogen oxides. The nitrogen oxide mass flow is in turn calculated on the basis of the rotational speed and/or the amount of fuel injected.